EARTHQUAKE. Earthquake denotes a shaking of the earth's crust, due to the passage of elastic waves, depending on some internal cause. These waves are principally of two classes: longitudinal, in which the particles vibrate as in sound; and transverse, in which they vibrate as in light; but near the origin of disturbance both classes of vibrations coexist and the movement of the earth particle is a combination of the two oscillations. A cubical element of the earth's crust has its volume com preSsed and its form distorted, with the pas sage of each successive wave and the rapid recurrence of this movement constitutes the trembling called an earthquake. The velocity of this wave propagation varies according to the elasticity and average density of the rock, the observed values lying between a few hun dred feet and about five miles per second, as in the recent San Francisco earthquake, which was propagated across the continent with great speed. Powerful earthquakes are now recorded all over the world by means of very sensitive seismographs and it is found that the greater the distance of propagation the more the waves are separated, the compressional waves out pacing the distortional ones.
The record of a South American earthquake, like that of Valparaiso, 16 Aug. 1906, occupies from two to three hours when registered upon seismographs in the United States and Europe ; and moreover the further the disturbance is prop agated the slower the earth movement becomes, so that from a violent agitation near the origin it becomes a gentle swaying of very slow char acter in the distance and may not be felt by men nor animals, though faithfully recorded on delicate long-distance seismographs. As more than 60 world-shaking earthquakes occur each year, one of these disturbances on the average is due each week and by the investigation of the propagation of these waves through the earth seismologists have drawn important con clusions regarding the constitution of the globe. In this way Mr. R. D. Oldham shows (Quar terly Journal of Geological Society, 1906) that the earth is fairly homogeneous throughout until we reach a distance of about 0.4 of the
radius from the centre, when a change takes place, the cause of which is not yet understood.
Moreover, it is possible for a seismologist to calculate from his record of an earthquake how far off the disturbance was. The em pirical formula given by Professor Omori of the Imperial University at Tokio, for very distant earthquakes is as follows (Publi cations Astron. Soc. Pacific, 10 June 1906) : where X, in kilometers, denotes the actual dis tance between the centre of disturbance and the place of observation ; and Y, in seconds, the total duration of the first and second prelim inary tremors. When the distance is small the formula becomes Xmn.--7.27Ysee-381an.; From the records obtained at the Lick Observa tory Professor Omori calculates that the most central part of the San Francisco earthquake was 80 or 90 miles to the west of Mount Ham ilton, which places the origin under the margin of the Pacific Ocean.
The motion of the earth particle usually is only a small fraction of an inch and houses resting on solid ground do not suffer greatly; but where the land is made by the filling in of soft earth it remains loose and by the pas sage of the earthquake waves is thrown into billows like the sea, so that all structures on such foundations are frequently destroyed. This was observed to be the effect at Lisbon as long ago as 1755 and illustrated again very clearly at San Francisco and at Valparaiso in 1906. Even the steel frames of brick buildings were destroyed on the made land at Valparaiso, though houses standing on rock foundation suf fered but little. Observations by Professor Omori and the writer at San Francisco show that on the made land the oscillations did not exceed about three inches. Owing to the rapidity of vibrations of this amplitude, however, few structures are able to withstand them. But on hard ground, where the vibrations were about a quarter of an inch, the worst that would result would be a cracking of the plastering and perhaps of the walls.